Retroreflective structures have been incorporated into a wide variety of different articles in order to make such articles more visible under low light conditions. A typical retroreflective structure works by reflecting a substantial portion of incident light back towards the illumination source. As a result, the underlying article and/or imagery printed on the retroreflective structure are easily seen by an observer in or near the path of the reflected light. For example, one common application for retroreflective articles involves covering the surface of a traffic sign with a layer of retroreflective sheeting. At night, incident light from the headlights of an oncoming motor vehicle bathes the sign in light, which is then reflected back to the driver of the motor vehicle. Information on the sign is easily read as a result.
Of course, traffic signs are not the only application for retroreflective products. Other examples of articles that benefit from retroreflective characteristics include lens elements on pavement markers (particularly raised pavement markings), street name signs, pavement marking tapes, reflectors on bicycles, conspicuity markings for motor vehicles and trains, traffic cones, license plates, self adhesive stickers (such as the validation stickers that are affixed to license plates or windshields), commercial advertising signs, clothing, barrels, barricades, and the like.
Many varieties of retroreflective structures are known, including cube corner retroreflective sheeting, encapsulated lens retroreflective sheeting, and enclosed lens retroreflective sheeting. Cube corner retroreflective sheeting typically uses a multitude of cube corner elements to retroreflect incident light. These structures generally comprise a planar sheet having a front surface which is intended to be seen by an observer and an array of cube corner elements provided on the backside of the planar sheet. The cube corner elements commonly are trihedral structures having a base proximal to the planar sheet and three mutually perpendicular lateral faces extending away from the sheet and meeting in a single corner, i.e., a cube corner.
In use, cube corner retroreflective sheeting is arranged so that the front surface faces the anticipated direction of the intended observers. A light ray incident upon the front surface enters a cube corner element, strikes one lateral face, is reflected to a second lateral face, and is again reflected from that second lateral face to pass out of the sheeting through the planar sheet back toward the direction of the source of the incident light. The incident light ray is reflected at the lateral faces due to total internal reflection and/or due to reflective coatings provided on the lateral faces. Illustrative examples of retroreflective cube corner elements are well known and have been described in U.S. Pat. No. 3,712,706 (Stamm); U.S. Pat. No. 4,025,159 (McGrath), U.S. Pat. No. 4,202,600 (Burke et al.), U.S. Pat. No. 4,243,618 (Van Arnam), U.S. Pat. No. 4,349,598 (White), U.S. Pat. No. 4,576,850 (Martens), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appeldorn et al.), U.S. Pat. No. 4,895,428 (Nelson et al.), and U.S. Pat. No. 5,117,304 (Huang et al.), all of which are incorporated herein by reference in their entirety.
Typical enclosed lens structures are shown in FIGS. 2 and 3 of Bailey et al., U.S. Pat. No. 4,664,966, the entire disclosure of which is incorporated herein by reference. There, a monolayer of glass microspheres are embedded within a polymeric binder having a surface intended to face an illumination source and a backside surface bearing a reflective coating. An adhesive may be used to adhere the structure to a suitable substrate.
A representative encapsulated lens structure is shown in McGrath, U.S. Pat. No. 4,025,159, incorporated herein by reference. There, a monolayer of glass microspheres is partially embedded in a base sheet. A cover sheet overlies the protruding surfaces of the spheres, forming a desirable air gap between the cover sheet and the spheres. A reflective coating also is provided underneath embedded surfaces of the spheres. An adhesive may be used to adhere the structure to a suitable substrate.
On many retroreflective articles, the exposed front surface that faces the direction from which the article is to be viewed are formed from thermoplastic and/or thermosetting polymers. Examples of polymer materials used to form these structures include thermosetting or thermoplastic polycarbonate, poly(meth)acrylate (PMMA), ethylene/acrylic acid copolymers (EAA), polyvinyl chloride (PVC), polyurethane, polyester, polyamide, polyimide, phenoxy, phenolic resin, cellulosic resin, polystyrene, styrene copolymer, epoxy, and the like. Many of these thermoplastic and thermosetting polymers have excellent rigidity or flexibility (depending upon the desired application), dimensional stability, and impact resistance, but unfortunately have poor abrasion resistance. Consequently, retroreflective articles incorporating these materials are susceptible to scratches, abrasion, and similar damage.
To protect the surfaces of retroreflective articles from physical damage, a tough, abrasion resistant "hardcoat" layer may be coated onto one or more portions of the retroreflective surfaces of such articles. Many previously known hardcoat layers incorporate a binder matrix formed from radiation curable prepolymers such as (meth)acrylate functional monomers. Such hardcoat compositions have been described, for example, in Japanese patent publication JP02-260145, U.S. Pat. No. 5,541,049, and U.S. Pat. No. 5,176,943. One particularly excellent hardcoat composition is described in WO 96/36669 A1. This publication describes a hardcoat formed from a "ceramer" used, in one application, to protect the surfaces of retroreflective sheeting from abrasion. As defined in this publication, a ceramer is a hybrid, polymerizable composite (preferably transparent) having inorganic oxide components, e.g., silica, of nanometer dimensions dispersed in an organic binder matrix.
Many ceramers are derived from aqueous sols of inorganic colloids according to a process in which a radiation curable binder matrix precursor (e.g., one or more different radiation curable monomers, oligomers, and/or polymers) and other optional ingredients (such as surface treatment agents that interact with the colloids of the sol, surfactants, antistatic agents, leveling agents, initiators, stabilizers, sensitizers, antioxidants, crosslinking agents, and crosslinking catalysts) are blended into the aqueous sol. The resultant composition is then dried to remove substantially all of the water. The drying step sometimes is referred to as "stripping". An organic solvent may then be added, if desired, in amounts effective to provide the composition with viscosity characteristics suitable for coating the composition onto the desired substrate. After coating, the composition can be dried to remove the solvent and then exposed to a suitable source of energy to cure the radiation curable binder matrix precursor.
The manufacture of ceramer compositions can be challenging due to the extremely sensitive characteristics of the colloids of the aqueous sol. Particularly, adding other ingredients, such as binder matrix precursors or other additives, to such sols tends to destabilize the colloids, causing the colloids to flocculate, i.e., precipitate out of the sol. Flocculation is not conducive to forming high quality coatings. Firstly, flocculation results in local accumulations of particles. These accumulations are typically large enough to scatter light which results in a reduction of the optical clarity of the resultant coating. Additionally, the accumulation of particles may cause nibs and/or other defects in the resultant coatings. In short, flocculation of the colloids causes the resultant ceramer composition to be cloudy, or hazy, and thus, coatings formed from the ceramer composition would be cloudy or hazy as well. Conversely, if colloid flocculation were to be avoided, the resultant ceramer composition would remain optically clear, allowing coatings comprising the ceramer composition to be optically clear as well.
Thus, the manufacture of ceramer compositions may require special processing conditions that allow binder precursors and/or additives to be incorporated into a sol to avoid colloid flocculation. Unfortunately, the processing conditions developed to manufacture one ceramer composition are often not applicable to the manufacture of a ceramer comprising different components.
One method of manufacturing ceramers from aqueous, colloidal sols involves incorporating one or more N,N-disubstituted (meth)acrylamide monomers, preferably N,N-dimethyl (meth)acrylamide (hereinafter referred to as "DMA"), into the binder matrix precursor. The presence of such a radiation curable material advantageously stabilizes the colloids, reducing the sensitivity of the colloids to the presence of other ingredients that might be added to the sol. By stabilizing the colloids, the presence of materials like DMA makes ceramers easier to manufacture. In addition to enhancing colloid stability, DMA provides other benefits. For example, ceramer compositions comprising DMA show better adhesion to polycarbonate and/or acrylic substrates and better processability as compared to otherwise identical ceramer compositions lacking DMA.
Unfortunately, the use of DMA also has some drawbacks. DMA is a "soft" monomer. That is, incorporating DMA into a ceramer tends to reduce the hardness and abrasion resistance of the ceramer. Additionally, DMA is also basic in nature. As a consequence, a ceramer composition incorporating DMA tends to attract and/or bind with acidic contaminants (coffee, soda pop, citrus juices, and the like) in the environment. Thus, ceramers incorporating DMA tend to be more vulnerable to staining.
Accordingly, it would be desirable to find an alternative approach for making ceramers without DMA, or with reduced amounts of DMA, such that (1) the colloids are sufficiently stable during ceramer manufacture, (2) the resultant ceramer is stain resistant, and/or (3) the resultant ceramer retains excellent hardness and abrasion resistance.
Fluorochemicals have low surface energy characteristics that would satisfy at least one of the aforementioned criteria. Specifically, because compositions with lower surface energy generally tend to show better stain resistance, the incorporation of a fluorochemical into a ceramer would be likely to enhance the ceramer's stain resistance. Unfortunately, the incorporation of fluorochemicals into a ceramer sol is extremely difficult. For example, because fluorochemicals are both hydrophobic (incompatible with water) and oleophobic (incompatible with nonaqueous organic substances), the incorporation of a fluorochemical into a ceramer sol often results in phase separation, e.g., colloid flocculation. This undesirable colloid flocculation can also result during the stripping process, i.e., when water is removed from the blended aqueous sol. Finally, not only can fluorochemicals be incompatible with ceramer sols, but such materials also tend to be "soft" in the sense that their presence in a coating adversely affects hardness and abrasion resistance.
Consequently, it would further be desirable to find a way to provide ceramers with good stain resistance using fluorochemicals or other stain resistant additives, while avoiding compatibility and hardness problems generally associated with fluorochemicals.
Light transmission to and from a retroreflective article, such as a traffic sign, can be impaired by water droplets on the surface of the traffic sign. A prominent form of precipitation that affects light transmission from traffic signs in particular is dew formation. Dew formation can be particularly problematic, because dew condenses onto signs predominantly at nighttime when the illuminating characteristics of retroreflective sheetings are most beneficial. Water droplets on traffic signs can significantly alter the path of incident and retroreflected light. This can make information on the sign much more difficult for passing motorists to read.
Thus, the elimination or reduction of small beaded water droplets on the surface of a sign increases retroreflectance and readability by reducing the extent to which incident light is scattered or otherwise misdirected by water droplets on the surface of a sign.
To hamper water droplet formation in moist conditions, coatings have been applied to signs to evenly spread the water over the coating. Water-spreading coatings typically include inorganic particles and may also include an organic binder.
For example, U.S. Pat. No. 4,576,864 to Krautter et al. discloses a water-spreading layer that is composed of colloidal particles of a metal or silicon oxide in which the water-spreading layer is adhered to a plastic substrate by an adhesive; U.S. Pat. No. 4,478,909 to Taniguchi et al. and U.S. Pat. No. 5,134,021 to Hosono et al. discloses an anti-fogging coating having finely divided silica particles dispersed in a matrix of polyvinyl alcohol and an organosilicon alkoxy compound or hydrolysates thereof, U.S. Pat. No. 4,409,285 to Swerdlow discloses a water-spreading coating comprising a mixture of large and small inorganic particles comprising colloidal silica and/or alumina; U.S. Pat. No. 4,481,254 to Fukishima et al. discloses an agricultural plastic film comprising an olefin resin and an amorphous hydrated aluminum silicate gel; U.K. Patent Application GB 2,249,041A to the Imperial College of Science, Technology and Medicine, discloses a modified hydrophobic plastic surface that has been subjected to an oxidation treatment and has a surface layer of colloidal hydrous metal oxide; Japanese Patent Kokai Publication No. HEI-3-50288 to Yamagishi et al. discloses an anti-fogging composition comprising a mixture of positively charged colloidal silica and alumina particles with a water-soluble aluminum salt and a nonionic surfactant; and U.S. Pat. Nos. 5,073,404, 4,844,946 and 4,755,425 to Huang disclose a retroreflective sheeting that has a transparent coating comprising colloidal silica and a polymer selected from aliphatic polyurethanes, polyvinyl chloride copolymers and acrylic polymers.
Other water-spreading layers are known that do not require inorganic particles. For example, U.S. Pat. No. 5,244,935 to Oshibe et al. discloses an ultraviolet curable anti-fogging composition agent comprising an acrylate or acrylamide block copolymer having a hydrophilic polymer segment and a hydrophobic polymer segment, a photopolymerizable compound, and a photoinitiator. The photopolymerizable compound has the formula CH.sub.2 .dbd.CHCOO(CH.sub.2 CRHO).sub.n OCCR.dbd.CH.sub.2 ; when n=0, anti-fogging properties were not exhibited and when n&gt;30, the resulting film was weak. U.S. Pat. No. 5,316,825 to Nakai et al. discloses an anti-fogging film made of a transparent synthetic resin having micro concavities of at most 10 .mu.m in depth and 20 .mu.m in width.
Other workers have reported that anti-fogging properties can be imparted to glass or surface-activated plastic substrates by reacting the substrate surfaces with silanol or siloxane-functionalized polymers or fluoropolymers. European Patent Application No. 0 620 255 A1 to Luckey, Ltd. reports that anti-fogging coatings can be produced from mixtures of an epoxy functionalized organosiloxane, an amino functionalized organosiloxane, a hydrophilic methacrylate monomer, and a curing catalyst. U.S. Pat. No. 5,270,080 to Mino et al. discloses anti-fogging compositions composed of silanol-functionalized fluoropolymers. European Patent Application Nos. 0 491 251 A1 and 0 492 545 A2 to Matsushita Electric Industrial Co. report water-repelling, oil-repelling anti-fogging films that are made from siloxy-functionalized hydrophobic compounds. These references report that plastic surfaces can be made reactive to hydroxyl groups or hydrophilic by corona treating the surface.
Other techniques have resulted in heterogeneous surfaces. U.S. Pat. No. 4,536,420 to Rickert discloses a water-wettable coating made from a mixture of colloidal acrylic resin and colloidal silica which, when cured, has a mud-cracked pattern, thus providing canals in the surface which tend to break up water droplets. Japanese Kokai Patent Publication 59-176,329 to Mitsubishi Monsanto Kasei Vinyl K.K. discloses transparent molded materials having patterned surfaces of hydrophilic and hydrophobic areas. In the examples, a patterned hydrophobic material is printed onto a hydrophilic film.
Some retroreflective articles are formed through embossing techniques in which information is embossed onto an article comprising retroreflective sheeting covering a surface of the article. For example, license plate blanks comprise a retroreflective sheet bonded to an underlying substrate. License plate numbers are formed in the blank using embossing techniques. Accordingly, if a ceramer coating is provided on retroreflective sheeting intended to be used in the manufacture of license plates, the ceramer must have sufficient stretchability (i.e., toughness or impact resistance) to be embossed without cracking. Stretchability of ceramer coatings is also important in other applications in which the retroreflective article is to be attached to an irregular surface or that will be subjected to flexing stresses during use. For example, a ceramer coating must have stretchability if the corresponding retroreflective article is to be attached to a traffic cone or to a curved sidewall of a motor vehicle. Additionally, retroreflective sheeting used in a lens of a raised pavement marker must be sufficiently stretchable to withstand flexing stresses when contacted by vehicle tires traversing, impacting, or hitting the pavement marker.
In addition, depending upon the intended use, it is highly desirable if the surface of a retroreflective article is receptive to printing with some kinds of inks and colorants to create graphic patterns on articles such as license plates, stickers, emblems, and the like. Yet, it is also desirable if these same articles are stain resistant at the same time so that undesired inks and colorants, e.g., graffiti, can be easily removed.